The invention relates generally to mass spectrometry, and more particularly to ion guides for use with mass spectrometers.
Many types of mass spectrometers are known, and are widely used for trace analysis and for determining the structure of ions. These spectrometers usually separate ions based on their mass-to-charge ratio (“m/z”). Some of these spectrometers include quadrupole mass analyzers, in which RF/DC ion guides are used for transmitting ions within a narrow range of m/z values; magnetic sector analyzers, in which large magnetic fields exert forces perpendicular to the direction of motion of moving ions, in order to deflect the ions according to their m/z; and time-of-flight (“TOF”) analyzers, in which measurement of flight time over a known path for an ion allows the determination of its mass-to-charge ratio.
Unlike quadrupole mass-analyzers, TOF analyzers can record complete mass spectra without the need for scanning parameters of a mass filter, thus providing a higher duty cycle and resulting in better use of the sample. In certain mass spectrometers, RF ion guides are coupled with orthogonal TOF mass analyzers, where the ion guide is for the purpose of transmitting ions to the TOF analyzer, or are used as collision cells for producing product ions and for delivering the product ions (in addition to any remaining precursor ions) to the TOF analyzer. Combining an ion guide with the orthogonal TOF is a convenient way of delivering ions to the TOF analyzer for analysis.
It is presently known to employ at least two modes of operation of orthogonal TOF mass spectrometers employing ion guides.
In the first mode, a continuous stream of ions leaves a radio-frequency-only quadrupole ion guide and is directed to an extraction region of the TOF analyzer. The stream is then sampled by TOF extraction pulses for detection in the normal TOF manner. This mode of operation has duty cycle losses as described, for example, in a tutorial paper by Chernushevich et al., in the Journal of Mass Spectrometry, v. 36, pp. 849-865, 2001 (“Chernushevich et al.”).
The second mode of operation is described in Chernushevich et al., as well as in U.S. Pat. No. 5,689,111 and in U.S. Pat. No. 6,285,027. This mode involves pulsing ions out of a two-dimensional ion guide such that ions having particular m/z values (i.e., m/z values within narrowly-defined ranges) are bunched together in the extraction region of the TOF. This mode of operation reduces transmission losses between the ion guide and the TOF, but due to the dependence of ion velocity on the m/z ratio only ions from a small m/z range can be properly synchronized, leading to a narrow range of m/z (typical (m/z)max/(m/z)min˜2) that can be effectively detected by the TOF analyzer. Thus, when ions with a broad range of masses have to be recorded, it is necessary to transmit multiple pulses having parameters specific to overlapping m/z ranges in order to record a full spectrum. This results in inefficiencies since ions outside the transmission window are either suppressed or lost. One way to avoid this loss is proposed in commonly-assigned U.S. Pat. No. 6,744,043. In this patent, an ion mobility stage is employed upstream of the TOF analyzer. The mobility migration time of the ions is somewhat correlated with the m/z values of the ions. This allows for adjustment of TOF window in pulsed mode so that the TOF window is always tuned for the m/z of ions that elute from the ion mobility stage. However, addition of the mobility stage to the spectrometer apparatus increases the complexity and cost of the apparatus. Moreover, the use of pulsed ejection and corresponding continual adjustment of the TOF window prevents optimal efficiencies in cycle time, or process turnaround, for the spectrometer.